i have a question im trying to get the achievement for impact guards and i want to know if i can do it in versus me versus the computer or do i have to do it in story mode or the tower mode ?

Only online or versus a computer via single player modes.

Just make a custom character and give them the skill "Auto Impact" if you have trouble doing them. Later on when you get the gear to up stats enough you can do the same thing to get the "Just impact" acheivement.

None of the acheivements can be unlocked via local versus. They can be unlocked in unranked online versus. This helps to get the "win with all armor broken" and "break all ooponents armor" acheivements, if you have a buddy with the game and live. Im usually not about milking acheivements online but ive never played against someone that blocked enough for me to break every peice of armor before they died, so that was the only way I could get it.

Just to add to what AcidicVision said, build a character with a lot of extra health, and the skill for gaining health from impacts. Try doing the higher levels of the Tower. Those characters will attack much more frequently, thus increasing the chance of getting the impacts. :)